Insulating oil

ABSTRACT

An insulating oil for use in electrical devices, comprising a mixture of trixylenylphosphate and 10% to 40% by volume of insulating oil having a viscosity of 5 cp to 15 cp at 30°C and selected from the group consisting of mineral oil, alkylbenzene, alkylnaphthalene, diallylalkane, 2-diphenylmethylether, 2-diphenyl-isopropylether, paraffin hydrocarbon oil and naphthene hydrocarbon oil.

BACKGROUND OF THE INVENTION

This invention relates to insulating oil compositions used in oil-filled or oil-immersed electrical devices.

Insulating oil is widely used in various types of electrical devices such as power cables, transformers, capacitors and so on. It has hithereto been customary to use mineral oil or diphenyl chloride as the insulating oil for such electrical devices. Recently, such electrical devices tend to become of greater capacity and more capable of withstanding higher voltages, so that the insulating oil used in them must have characteristics to meet the tendncy.

Mineral oil alone, however, is combustible and inferior in such electrical characteristics as dielectric constant and visible gas generating voltage. While diphenyl chloride is incombustible and superior in the above-mentioned characteristics, it is very toxic to the human body so that the use of this compound is undesirable from the view point of environmental pollution.

Accordingly, the primary object of the invention is to provide an insulating oil composition which has good characteristics as such oil.

Another object of the invention is to provide an insulating oil composition which is superior to mineral oil in electrical characteristics such as dielectric constant, visible gas generating voltage, etc.

Another object of the invention is to provide such an insulating oil composition as aforesaid which is incumbustible and has no toxicity to the human as well as animal life.

In accordance with the invention there is provided an insulating oil composition which comprises a mixture of trixylenylphosphate (which will be referred to as TXP) and an insulating oil having a viscosity of 5 to 15 centipoises (which will be abbreviated to cp) at 30°C, the amount of of the latter oil to be mixed with TXP being 10% to 40% by volume of the mixture. The insulating oil which has a viscosity of 5 cp to 15 cp can be selected from the following group: mineral oil (to be referred to as MO), alkylbenzene (to be referred to as AB), alkylnaphthalene (to be referred to as AN), diallylalkane (to be referred to as DAA), paraffin hydrocarbon oil (to be referred to as PO), naphthene hydrocarbon oil (to be referred to as NO), 2-diphenylmethylether (to be referred to as DME), 2-diphenylisopropylether (to be referred to as DPE), etc.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

The combustibility and electrical characteristics of TXP and the above-mentioned insulating oils are given in TABLE 1.

                                      TABLE 1                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                             Specific                                                                              Flash                                                            (cp)   gravity                                                                               point                                                                              Combustibility                                               (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating                                                                     oil                                                                            TXP      103    1.12   210 Incombustible                                       MO       9      0.87   132 Combustible                                         AB       10     0.88   136 "                                                   AN       8      0.96   140 "                                                   DAA      8      0.96   150 "                                                   PO       9      0.86   132 "                                                   NO       9      0.88   136 "                                                   DME      13     0.98   134 "                                                   DPE      14.7   0.98   139 "                                                   Characteristics                                                                         Dielectric                                                                            Dielectric                                                                            Volume                                                                               Visible gas                                                constant                                                                              loss tangent                                                                          resistivity                                                                          generating                                                 (at 80°C)                                                                      (%)    (Ω-cm)                                                                         voltage                                                           (at 80°C)                                                                            (kV/mm)                                           __________________________________________________________________________     Insulating                                                                     oil                                                                            TXP      5.99   0.21   1.0×10.sup.14                                                                  57.0                                              MO       2.20   0.010  1.5×10.sup.15                                                                  47.0                                              AB       2.20   0.010  2.0×10.sup.15                                                                  52.0                                              AN       2.47   0.010  2.5×10.sup.15                                                                  65.0                                              DAA      2.51   0.010  1.0×10.sup.15                                                                  70.0                                              PO       2.20   0.010  2.5×10.sup.15                                                                  40.0                                              NO       2.21   0.010  2.0×10.sup.15                                                                  45.0                                              DME      3.80   0.06   1.0×10.sup.15                                                                  68.5                                              DPE      4.18   0.06   1.0×10.sup.15                                                                  67.0                                              __________________________________________________________________________

The physical properties such as viscosity, etc, as well as the electrical characteristics such as dielectric constant, visible gas generating voltage, etc of mixtures comprising TXP and 10%, 20%, 30% by volume, respectively, of one or two of the above insulating oils are given in TABLES 2 through 12 in comparison with those of the mixtures comprising TXP and 45% and 50% of one or two of those insulating oils.

In TABLE 2 the material tested comprises a mixture of TXP and MO; in TABLE 3 it comprises a mixture of TXP and AB; in TABLE 4 it comprises a mixture of TXP and AN; in TABLE 5 it comprises a mixture of TXP and DAA; in TABLE 6 it comprises a mixture of TXP and PO; in TABLE 7 it comprises a mixture of TXP and NO; in TABLE 8 it comprises a mixture of TXP and DME; in TABLE 9 it comprises a mixture of TXP and DPE; in TABLE 10 it comprises a mixture of TXP, MO and AN; in TABLE 11 it comprises a mixture of TXP, AN and DME; and in TABLE 12 it comprises a mixture of TXP, MO and DME.

In the above TABLES one or two of the insulating oils are mixed with TXP. If three or more of the insulating oils are mixed with TXP, similar results are obtained to those obtained from the mixture of TXP and one or two of the insulating oils, as will be obvious to those skilled in the art.

The flash point has been measured in the following manner. The oil whose flash point is to be tested is put in a pot, which is tightly closed. The oil is slowly heated while it is being stirred at a constant speed. A thermometer is provided to measure the temperature of the oil in the pot. When the oil approaches the expected flash point, the stirring is stopped whenever the temperature rises a predetermined number of degrees, say, 2°C, whereupon a small window provided in the lid of the pot is opened so that a 7 mm long flame is put in the pot so as to see if the vapor of the oil is ignited. The lowest temperature at which the vapor is ignited is the flash point of that oil.

The combustibility has been tested in the following manner. A clean glass sleeve about 5 mm in diameter and about 20 mm in length is dipped in the insulating oil to be tested for one minute, after which it is taken out of the oil so as to be held horizontal for one minute. Then a flame having a length of about 5 mm is applied to the sleeve for three seconds, after which the flame is moved away from the sleeve so as to see if there is a burning flame on the sleeve. In the TABLES, the word "combustible" means that there was a burning flame under the above condition while the word "incombustible" means that there was no such burning flame.

The visible gas generating voltage has been measured in the following manner. A voltage is applied to a sheet of paper immersed with the insulating oil to be tested and the voltage is increased until visible gas is generated. The level of the impressed voltage at which visible gas is generated is the visible gas generating voltage. It is well known that as the voltage impressed on the oil-immersed sheet of paper is increased, visible gas is generated immediately before breakdown occurs. Therefore the visible gas generating voltage can be taken as a measure of breakdown of the insulation of the oil-immersed paper.

                                      TABLE 2                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                              Specific                                                                             Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  MO                                                                       90    10 78     1.09   165 Incombustible                                       80    20 63     1.07   155 "                                                   70    30 48     1.04   151 "                                                   60    40 38     1.02   146 "                                                   55    45 34     1.00   144 Combustible                                         50    50 30     0.99   142 "                                                   Characteristics                                                                         Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                          __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  MO                                                                       90    10 5.07   0.21   1.0×10.sup.14                                                                   57.0                                             80    20 4.85   0.20   1.5×10.sup.14                                                                   57.0                                             70    30 4.38   0.15   3.9×10.sup.14                                                                   56.5                                             60    40 3.92   0.12   7.0×10.sup.14                                                                   56.2                                             55    45 3.70   0.10   9.0×10.sup.14                                                                   55.8                                             50    50 3.50   0.08   9.0×10.sup.14                                                                   55.2                                             __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                             Specific                                                                              Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  AB                                                                       90    10 79     1.09   168 Incombustible                                       80    20 63     107    160 "                                                   70    30 52     1.04   153 "                                                   60    40 40     1.02   148 "                                                   55    45 34     1.00   148 Combustible                                         50    50 32     0.94   145 "                                                   Characteristics                                                                         Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                          __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  AB                                                                       90    10 5.06   0.21   1.0×10.sup.14                                                                   57.0                                             80    20 4.85   0.20   1.3×10.sup.14                                                                   57.0                                             70    30 4.38   0.16   4.0×10.sup.14                                                                   56.8                                             60    40 3.92   0.13   7.2×10.sup.14                                                                   56.2                                             55    45 3.70   0.10   8.5×10.sup.14                                                                   56.5                                             50    50 3.50   0.08   9.5×10.sup.14                                                                   56.0                                             __________________________________________________________________________

                                      TABLE 4                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                              Specific                                                                             Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating oil                                                                 % volume ratio                                                                  TXP  AN                                                                       90    10 75     1.11   173 Incombustible                                       80    20 60     1.09   163 "                                                   70    30 47     1.07   158 "                                                   60    40 36     1.05   154 "                                                   55    45 32     1.04   153 Combustible                                         50    50 28     1.03   151 "                                                   Characteristics                                                                         Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                          __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  AN                                                                       90    10 5.25   0.20   1.0×10.sup.14                                                                   58.0                                             80    20 5.05   0.20   1.8×10.sup.14                                                                   58.5                                             70    30 4.58   0.17   4.0×10.sup.14                                                                   59.5                                             60    40 4.15   0.13   7.5×10.sup.14                                                                   61.0                                             55    45 3.90   0.10   8.8×10.sup.14                                                                   61.5                                             50    50 3.80   0.09   9.0×10.sup.14                                                                   62.0                                             __________________________________________________________________________

                                      TABLE 5                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                             Specific                                                                              Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  DAA                                                                      90   10  75     1.10   175 Incombustible                                       80   20  62     1.09   164 "                                                   70   30  48     1.07   160 "                                                   60   40  35     1.05   152 "                                                   55   45  31     1.03   150 Combustible                                         50   50  28     1.01   150 "                                                   Characteristics                                                                         Dielectric                                                                           Dielectric                                                                            Volume Visible gas                                                constant                                                                             loss tangent                                                                          resistivity                                                                           generating                                                 (at 80°)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                           __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  DAA                                                                      90   10  5.26  0.20   1.0×10.sup.14                                                                   60.0                                              80   20  5.06  0.20   2.0×10.sup.14                                                                   59.0                                              70   30  4.58  0.20   4.0×10.sup.14                                                                   61.5                                              60   40  4.16  0.14   7.0×10.sup.14                                                                   62.5                                              55   45  3.81  0.09   8.0×10.sup.14                                                                   63.0                                              50   50  3.57  0.07   8.5×10.sup.14                                                                   63.5                                              __________________________________________________________________________

                                      TABLE 6                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                              Specific                                                                             Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  PO                                                                       90    10 76     1.04   170 Incombustible                                       80    20 64     1.04   160 "                                                   70    30 47     1.03   156 "                                                   60    40 35     1.00   148 "                                                   55    45 32     0.98   144 Combustible                                         50    50 30     0.97   142 "                                                   Characteristics                                                                         Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°)                                                                              (kV/mm)                                          __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  PO                                                                       90    10 5.18   0.20   1.0×10.sup.14                                                                   57.0                                             80    20 4.96   0.20   2.0×10.sup.14                                                                   56.5                                             70    30 4.50   0.14   3.5×10.sup.14                                                                   56.0                                             60    40 4.04   0.12   8.0×10.sup.14                                                                   55.0                                             55    45 3.62   0.10   9.0×10.sup.14                                                                   53.0                                             50    50 3.41   0.08   9.5×10.sup.14                                                                   51.0                                             __________________________________________________________________________

                                      TABLE 7                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                             Specific                                                                              Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating oil                                                                 % volume ratio                                                                  TXP  NO                                                                       90    10 76     1.10   172 Incombustible                                       80    20 65     1.08   165 "                                                   70    30 46     1.05   158 "                                                   60    40 34     1.03   148 "                                                   55    45 32     0.95   146 Combustible                                         50    50 30     0.91   144 "                                                   Characteristics                                                                         Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                          __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  NO                                                                       90    10 5.18   0.20   1.0×10.sup.14                                                                   57.5                                             80    20 4.98   0.20   1.0×10.sup.14                                                                   57.0                                             70    30 4.50   0.15   3.5×10.sup.14                                                                   56.5                                             60    40 4.04   0.12   8.0×10.sup.14                                                                   55.0                                             55    45 3.62   0.10   9.0×10.sup.14                                                                   54.0                                             50    50 3.41   0.08   1.0×10.sup.15                                                                   53.0                                             __________________________________________________________________________

                                      TABLE 8                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                             Specific                                                                              Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP                                                                                DME                                                                       90  10   80     1.10   172 Incombustible                                       80  20   68.5   1.09   164 "                                                   70  30   55     1.07   158 "                                                   60  40   38     1.05   148 "                                                   55  45   30     1.03   144 Combustible                                         50  50   26     1.02   142 "                                                   Characteristics                                                                         Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                          __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP  DME                                                                      90   10  5.45   0.30   1.0×10.sup.14                                                                   60.0                                             80   20  5.20   0.25   2.0×10.sup.14                                                                   61.0                                             70   30  5.05   0.25   3.0×10.sup.14                                                                   61.5                                             60   40  4.60   0.20   7.5×10.sup.14                                                                   62.0                                             55   45  4.50   0.18   8.0×10.sup.14                                                                   64.0                                             50   50  4.40   0.17   8.5×10.sup.14                                                                   65.0                                             __________________________________________________________________________

                                      TABLE 9                                      __________________________________________________________________________     Characteristics                                                                         Viscosity                                                                             Specific                                                                              Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                             __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP DPE                                                                       90   10  83     1.10   172 Incombustible                                       80   20  74     1.08   168 "                                                   70   30  61     1.06   160 "                                                   60   40  43     1.05   150 "                                                   55   45  40     1.03   146 Combustible                                         50   50  38     1.02   142 "                                                   Characteristics                                                                         Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                          __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP DPE                                                                       90   10  5.75   0.31   1.0×10.sup.14                                                                   59.0                                             80   20  5.65   0.28   2.0×10.sup.14                                                                   59.0                                             70   30  5.50   0.24   3.5×10.sup.14                                                                   60.5                                             60   40  5.05   0.20   7.0×10.sup.14                                                                   61.0                                             55   45  4.85   0.18   8.5×10.sup.14                                                                   62.0                                             50   50  4.70   0.16   9.0×10.sup.14                                                                   63.0                                             __________________________________________________________________________

                                      TABLE 10                                     __________________________________________________________________________     Characteristics                                                                          Viscosity                                                                             Specific                                                                              Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                            __________________________________________________________________________     Insulating oil                                                                  & volume ratio                                                                  TXP                                                                              MO AN                                                                      90   5  5 77     1.09   166 Incombustible                                      80  10 10 62     1.07   162 "                                                  70  10 20 46     1.05   156 "                                                  60  20 20 37     1.03   150 "                                                  55  30 25 32     1.02   146 Combustible                                        50  20 30 28     1.01   145 "                                                  Characteristics                                                                          Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                         __________________________________________________________________________     Insulating oil                                                                  & volume ratio                                                                 TXP                                                                               MO AN                                                                      90   5  5 5.13   0.20   1.0×10.sup.14                                                                   57.5                                            80  10 10 4.91   0.20   1.5×10.sup.14                                                                   58.0                                            70  10 20 4.47   0.15   4.0×10.sup.14                                                                   59.0                                            60  20 20 4.07   0.12   7.0× 10.sup.14                                                                  58.0                                            55  30 25 3.85   0.10   9.0×10.sup.14                                                                   57.5                                            50  20 30 3.65   0.08   9.0×10.sup.14                                                                   58.0                                            __________________________________________________________________________

                                      TABLE 11                                     __________________________________________________________________________     Characteristics                                                                           Viscosity                                                                             Specific                                                                              Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                           __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP                                                                               AN IME                                                                     90   5  5  81     1.14   166 Incombustible                                     80  10 10  65     1.11   158 "                                                 70  10 20  54     1.07   154 "                                                 60  20 20  43     1.04   149 "                                                 55  30 25  38     1.02   147 Combustible                                       50  20 30  34     1.00   145 "                                                 Characteristics                                                                           Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                        __________________________________________________________________________     Insulating oil                                                                  & volume ratio                                                                 TXP                                                                               AN DME                                                                     90   5 5   5.44   0.28   1.5×10.sup.14                                                                   58.0                                           80  10 10  5.23   0.25   2.7×10.sup.14                                                                   57.5                                           70  10 20  4.98   0.24   3.2×10.sup.14                                                                   59.5                                           60  20 20  4.54   0.20   7.8×10.sup.14                                                                   61.5                                           55  30 25  4.26   0.16   8.3×10.sup.14                                                                   62.5                                           50  20 30  4.42   0.15   8.5×10.sup.14                                                                   62.5                                           __________________________________________________________________________

                                      TABLE 12                                     __________________________________________________________________________     Characteristics                                                                           Viscosity                                                                             Specific                                                                              Flash                                                                              Combustibility                                               (cp)   gravity                                                                               point                                                            (at 30°C)                                                                      (at 15°C)                                                                      (°C)                                           __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP                                                                               MO DME                                                                     90   5  5  82     1.12   165 Incombustible                                     80  10 10  68     1.09   156 "                                                 70  10 20  55     1.05   152 "                                                 60  20 20  44     1.03   147 "                                                 55  30 25  40     1.02   145 Combustible                                       50  20 30  35     0.99   142 "                                                 Characteristics                                                                           Dielectric                                                                            Dielectric                                                                            Volume Visible gas                                               constant                                                                              loss tangent                                                                          resistivity                                                                           generating                                                (at 80°C)                                                                      (%)    (Ω-cm)                                                                          voltage                                                          (at 80°C)                                                                             (kV/mm)                                        __________________________________________________________________________     Insulating oil                                                                 & volume ratio                                                                  TXP                                                                               MO DME                                                                     90   5  5  5.32   0.28   1.5×10.sup.14                                                                   57.0                                           80  10 10  5.03   0.25   2.5×10.sup.14                                                                   57.0                                           70  10 20  4.82   0.25   3.0×10.sup.14                                                                   57.0                                           60  20 20  4.32   0.20   7.8×10.sup. 14                                                                  56.0                                           55  30 20  4.02   0.17   8.5×10.sup.14                                                                   55.5                                           50  20 30  4.23   0.16   8.7×10.sup.14                                                                   56.0                                           __________________________________________________________________________

From the above TABLES it is apparent that the mixtures prepared in accordance with the invention have higher dielectric constants than that of mineral oil which is 2.20 at 80°C. Also the mixed oils of the invention have higher visible gas generating voltage than that of mineral oil which is 47.0 kV/mm. This means that the mixed oils prepared in accordance with the invention are superior to the conventional insulating oil comprising mineral oil alone with respect to dielectric constant and visible gas generating voltage.

With respect to combustibility, mineral oil is combustible and the mixtures which comprises TXP and 45% and 50%, respectively, of the insulating oils are also combustible. AB, AN, DAA and all the other insulating oils to be mixed with TXP are individually combustible, so that if the amount of the oil mixed with TXP exceeds 50% by volume, the resultant mixture will apparently become combustible. Therefore, the maximum volme of the insulating oil or oils to be mixed with TXP is determined to be 40% so as to prevent the resultant mixture from becoming combustible.

From the above TABLES it can be seen that as the volume of the insulating oil or oils to be mixed with TXP is increased, the dielectric constant of the resultant mixture becomes lower. Therefore the volume to be mixed with TXP should not be very large.

The experiments have disclosed that TXP alone is incombustible. However, the viscosity of the compound is as high as 76.0 cp, so that if it alone is used as insulating oil in an electrical device, problems occur due to its poor cooling effect and low degree of immersion.

Unlike diphenyl chloride, the mixed insulating oils of the invention have little toxicity, so that their use in electrical devices are desirable from the viewpoint of environmental pollution and sanitation.

As can be seen from the foregoing description, the insulating oil compositions of this invention are incombustible and superior to the conventional mineral oil in electrical characteristics such dielectric constant, visible gas generating voltage and have little or no toxicity, so that the oil compositions of the invention are highly valued in industrial use. 

What we claim is:
 1. An insulating oil composition for use in electrical devices, comprising trixylenylphosphate and from 10% to 40% by volume of insulating oil having a viscosity of 5 cp to 15 cp at 30°C.
 2. An insulating oil composition for use in electrical devices, comprising trixylenylphosphate and from 10% to 40% by volume of a single kind of insulating oil having a viscosity of 5 cp to 10 cp at 30°C.
 3. An insulating oil composition for use in electrical devices, comprising trixylenylphosphate and from 10% to 40% by volume of a mixture of a plurality of insulating oils having a viscosity of 5 cp to 15 cp at 30°C.
 4. The composition of claim 2, wherein said insulating oil is a mineral oil.
 5. The composition of claim 2, wherein said insulating oil is an alkylbenzene.
 6. The composition of claim 2, wherein said insulating oil is an alkylnaphthalene.
 7. The composition of claim 2, wherein said insulating oil is 2-diphenylmethylether.
 8. The composition of claim 2, wherein said insulating oil is 2-diphenylisopropylether.
 9. The composition of claim 2, wherein said insulating oil is a paraffin hydrocarbon oil.
 10. The composition of claim 2, wherein said insulating oil is a naphthene hydrocarbon oil.
 11. The composition of claim 3, wherein said plurality of insulating oils are selected from the group consisting of mineral oils, alkylbenzenes, alkylnaphthalenes, 2-diphenylmethylether, 2-diphenylisopropylether, paraffin hydrocarbon oils and naphthene hydrocarbon oils. 